A user's heart rate can be leveraged as an input to various health and fitness endeavors. It can be used to estimate energy expenditure, set exercise targets, determine stress, and estimate level of fitness. Several devices, among them wearable or portable devices, are used to determine a user's heart rate using optical signals. Some wearable devices incorporate optical heart rate monitors which do not require a chest strap. These wearable devices can provide a quick and easy method of estimating a user's heart rate under various conditions.
There may be interest in determining heart rate in various modes. For example, heart rate may be determined continuously, such as during exercise; heart rate may be determined in the background throughout a period of time or continually when the wearable device is on; and heart rate may be determined in response to a specific one-time user request.
In one example, heart rate may be estimated from the optical signal measured by a sensor such as a pulse oximeter using a technique called photoplethysmography (PPG). In other examples, a combination of LED and photodiode can be used in place of or in addition to a pulse oximeter. Heart rate estimates through most methods can be subject to noise from various sources. For example, motion severely degrades the quality of the optical signal resulting in inaccurate heart rate estimates when a user is not substantially still.